Oxygenated compounds, such as various alcohols and ethers, have gained widespread application as gasoline additives. Such compounds provide a source of oxygen in the fuel to promote more complete combustion, thereby reducing emissions of carbon monoxide and various hydrocarbons which survive incomplete combustion.
Alcohols such as methanol and ethanol have long been used as gasoline additives. These alcohols, however, have relatively poor blending synergies with gasoline, and fuel mixtures containing them are frequently prone to undesirable phase separation. This tendency to phase separate requires the use of further additives, such as tert-butyl alcohol (TBA), to inhibit phase separation. Other alcohols, such as isopropyl alcohol (IPA), have also been proposed (see, e.g., U.S. Pat. No. 5,191,129) but are not widely used.
Another oxygenated gasoline additive that has recently come into wide usage is methyl tert-butyl ether (MTBE). In addition to being a source of oxygen, MTBE has a relatively high octane number (105), and thus can increase the octane number of fuels as well as promote their cleaner burning. As a result of its octane-enhancing and oxygenating properties, MTBE has become a large tonnage oxygenate and is used worldwide. However, it is not clear that such widespread use of MTBE should continue, in light of safety and health concerns associated with its use as a gasoline additive. In addition, MTBE and related compounds such as ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) are relatively expensive to produce, since they require the use of methyl or ethyl alcohol which are not compounds conventionally produced in refinery processes.
Although alcohols and esters are currently used as oxygenating fuel additives, little or no attention has been focused on the possibility of using ketones as additives. As the present invention discloses, certain ketones can have very high octane numbers as well as favorable mixing properties with gasoline. For example, methyl isopropyl ketone (MIPK) has an octane number of 125. However, although MIPK is a known compound, no efficient, economically feasible preparation of MIPK, or of other branched aliphatic ketones, has previously been disclosed.
MIPK has been prepared in the laboratory by the AlCl.sub.3 catalyzed reaction of pivaloyl chloride with carbon monoxide in the presence of isopentane (Balaban and Nenitzescu, J. Liebigs Ann. Chem. 1959, 625, 66). MIPK has also been reported as a minor product in the AlCl.sub.3 catalyzed reaction of isobutane with carbon monoxide, which yields primarily pivalic acid and isobutyl tert-butyl ketone. An alternative preparation of MIPK was reported by Hart et al. by reacting isoamylene with trifluoroperacetic acid and boron trifluoride (Hart and Lerner, J. Org. Chem. 1967, 32 2669).
In general, Friedel-Crafts (i.e., aluminum halide) catalyzed conversion of saturated hydrocarbons with carbon monoxide gives complex mixtures of products, containing carboxylic acids and ketones. For example, U.S. Pat. No. 2,346,701 discloses a process of oxygenating propane by reacting it with carbon monoxide in the presence of an anhydrous aluminum halide. The reaction produces a complex and poorly characterized mixture of products, including ketones, aldehydes and carboxylic acids.
U.S. Pat. No. 2,874,186 discloses a process for carboxylating normal paraffins, isoparaffins and naphthenes to produce ketones, acids and esters. The process uses a BF.sub.3 /HF catalyst to oxygenate saturated hydrocarbons with carbon monoxide to produce mainly carboxylic acid products.
Similarly, U.S. Pat. No. 3,356,720 discloses a process for preparing ketones and carboxylic acids from saturated hydrocarbons using a Friedel-Crafts catalyst and a hydrocarbon substituted carbinyl halide.
These prior efforts to oxygenate aliphatic hydrocarbons make use of the classic Koch-type of acid catalyzed carbonylation chemistry. Koch chemistry is well known and is discussed, for example, in Olah, Friedel-Crafts and Related Chemistry, Wiley-Interscience, New York (1963), and Olah & Molnar, Hydrocarbon Chemistry, Wiley-Interscience, New York (1995), the disclosures of which are incorporated herein by reference. Products obtained in reacting branched alcohols or olefins with carbon monoxide in strong acids in Koch reactions are predominantly branched carboxylic acids with some by-products, such as secondarily formed oligomers and some carbonyl compounds. Many strong acid and even superacid catalysts, such as H.sub.2 SO.sub.4, HF, CF.sub.3 SO.sub.3 H, HF--BF.sub.3 and HF--SbF.sub.5 have found application in Koch chemistry. (For a review, see Olah et al., Superacids, Wiley-Interscience, New York, p. 295 (1985).) Branched saturated hydrocarbons were known as early as the 1930's to react with carbon monoxide in the presence of aluminum trichloride or other Friedel-Crafts catalysts, mainly through the work of Hopff and Nenitzescu, respectively. They also react in the presence of protic superacids, such as HF--BF.sub.3 (see U.S. Pat. No. 2,874,186) or HF--SbF.sub.5 (Paatz & Weisgerber, Chm. Ber. 1967, 100, 984) giving typical Koch-Haaf products: primarily branched carboxylic acids, with some oligomers and ketones formed as by-products. Conventional Koch chemistry, however, fails to selectively and efficiently produce branched aliphatic ketones without significant production of carboxylic acids.
Thus, there is a need for a new type of oxygenated gasoline component which can enhance the octane number of the fuel as well as promote cleaner combustion. Further, there is a need for methods to produce such oxygenated additives which are easily adapted for use in hydrocarbon refinery processes.